The present invention relates to a dehydrochlorination method based on the use of a dipolar aprotic solvent and a lithium halide under reflux conditions. More particularly, the present invention relates to the dehydrochlorination of 1,1,1-trichloro-2,2-bis(4-hydroxyphenyl)ethane to produce 1,1-dichloro-2,2-bis(4-hydroxyphenyl)ethylene.
As taught by S. Porejko and Z. Weilgosz, Synthesis and Properties of Polycarbonates with Chloro-bisphenols, Polymeri, 13 (2) 55 (1968) 1,1-dichloro-2,2-bis(4-hydroxyphenyl)ethylene of the formula, ##STR1## referred to hereinafter as "dichloride", can be used to make high molecular weight polycarbonate. The procedure recommended by Porejko et al for making the dichloride is based on the dehydrochlorination of the corresponding "trichloroethane dihydric phenol", or "trichloride", as shown by the following equation, ##STR2## The trichloride, as taught by Porejko et al, can be made by a standard technique based on the condensation of chloral, or its hydrated form with phenol. As used hereinafter, the term "trichloride" will signify a trichloroethane bisphenol of the formula, ##STR3##
Experience has shown that although the preparation of trichloride by Porejko et al followed by its dehydrochlorination, using a methanol solution of potassium hydroxide, by the method of Porejko et al, can be employed to make dichloride, the resulting dehydrochlorination reaction solids can contain in addition to the dichloride, unreacted trichloride, and various contaminants. The following contaminants have been found in the trichloride dehydrochlorination reaction mixture of Porejko et al: ##STR4## where X can be H or methyl.
FIG. 1 is a typical liquid chromatogram of a trichloride dehydrochlorination mixture made by Porejko et al. The dichloride absorbances of contaminants are also shown. Based on the use of a calibration curve obtained from using pure materials, the concentration of the trichloride having a retention volume of about 48 ml is about 3,200 ppm.
In Polish Pat. No. 144,756, Wielgosz et al taught that improved dichloride purity can be achieved, if 6-8 mols of KOH, per mol of trichloride instead of 15 mols of KOH per mol of trichloride is used for trichloride dehydrohalogenation. A temperature of 40.degree.-50.degree. C. is also recommended. However, FIG. 2, a liquid chromatogram of the dichloride made by Wielgosz et al's improved procedure still shows significant amounts of contaminants. The Wielgosz et al's dichloride has a dark color; it has an absorbance value of 1.24, as shown by measuring its absorbance in methanol solution (2.50 gm/50 ml in a 10 cm cell) using a Carey 14 recording spectrophotometer with light at 425 nm. In addition, as shown by FIG. 2, the trichloride concentration at 48 ml retention volume is about 3,600 ppm.
The present invention is based on the discovery that a more highly selective dehydrohalogenation of the trichloride can be achieved by the use of a refluxing dipolar aprotic solvent, such as dimethylformamide in the presence of a lithium halide catalyst, such as lithium chloride, followed by the addition of water to the reaction mixture to effect separation of substantially pure dichloride crystals. As used hereinafter, the expression "substantially pure" when referring to dichloride will signify a dichloride having an absorbance value of less than 0.3 when measured as described above. In addition, a substantially pure dichloride can provide a liquid chromatogram similar to FIG. 3, having less than 1000 ppm of trichloride which, except for dichloride, is substantially free of absorbances exceeding 60% and preferably 50% of the 0.1 AUFS (absorbance units full scale) when tested as follows:
A Waters Model 244 liquid chromatograph is used, equipped with a Model U6K injector, a .mu.Bondapak C.sub.18 column, a Model 440 detector equipped with a 10 mm cell and operated at 280 nm set at 0.1 AUFS and a 10 millivolt Houston Instrument Omniscribe recorder with a chart speed of 0.25 centimeters per minute. Ten microliters of 10% (wt/vol) methanol solution of the dichloride is injected into the column and it is eluted at 2 ml per minute, where the solvent mixture is programmed linearly over a 1 hour period from an initial composition of 40% methanol and 60% water to a final composition of 100% methanol.
Prior to the present invention, as shown by R. P. Holysz, JCS, 57 4432 (1353) dimethylformamidelithium chloride mixtures have been found to be effective for dehydrohalogenating fused ring systems for making 4-halo-3-keto steroids. More recently, O. R. Jackson et al, JCS, Perkin Trans II, 308 (1972) and McLellan et al, ibid, 1818 (1974), investigated the dehydrohalogenation of certain trichlorodiarylethanes using dimethylformamide and lithium chloride mixtures. Even though dimethylformamide and lithium chloride have been used to effect the removal of hydrogen chloride from various trichlorodiarylethanes, prior to the present invention such dehydrohalogenation was never attempted with a dihydric phenol, such as a trichloride of formula (2). Surprisingly, even though a significant degree of rearrangement would normally be expected, it has been found that only a minor amount of the dihydric phenol of formula (7) is generated during dehydrochlorination.